This invention relates to local area network collision detection, and more specifically to collision detection in a local area network employing a medium wherein each decentralized node or each control node is given an opportunity to examine all potentially interfering signals. A typical network in which the invention is used is a carrier band network or a fiber optic network, and a typical use is in a CSMA/CD (Carrier Sense Multiple Access/Collision Detection) channel as for example defined by IEEE Standard 802.3 (Ethernet). A particular application is in a star-type optical fiber network wherein all signals are funneled through a head end.
Serial data streams in a local area network system are typically encoded in a code wherein transitions are expected at discrete times, subject to a timing window of phase tolerance. One such code is the d.c.-free binary Manchester Code specified by IEEE Standard 802.3 wherein all data is conveyed in bit cells containing the complement of the value of the data bit in the first half of the bit cell and the value of the data bit in the second half of the bit cell. Thus a transition is always expected at the center of the bit cell.
Collision detection is needed in CSMA/CD channels in order to detect interfering signals in the channel so that transmission can be halted. Collision detection in a broadband channel, as distinguished from a baseband channel where d.c. can be detected, requires special provision. In particular in a fiber optic channel, a known technique for collision detection is by means of a pulse width detector operative for detecting the edge to edge width of a received pulse. U.S. Pat. No. 4,561,091 to Scholl et al. describes such collision detection. Under certain circumstances, for example when two optically-transmitted signals of high transition rate relative to the expected pulse width collide, the collision can go undetected because the pulse width of the signals in collision may not produce adequate pulse broadening.
Still other collision detection techniques are known for fiber optic signals in a CSMA/CA network. One such technique is disclosed in U.S. Pat. No. 4,560,984 to F. W. Scholl. Therein, each transmitted signal is accompanied by a high-amplitude collision signal such that a high-amplitude threshold detector can indicate a collision by sensing presence of two such collision signals within a timed period. Nevertheless, it is frequently undesirable to provide an in-band collision detection technique which relies on additional signals.
Other patents address collision detection in optical networks. U.S. Pat. No. 4,776,041 to C. R. Husbands describes a collision detection apparatus in a star network. U.S. Pat. No. 4,562,582 to Tokura et al. describes an AC-coupled CSMA/CD system using a partial response circuit wherein the collision decision is based on detected code-rule violations of the Manchester Code. U.S. Pat. No. 4,582,872 to Bhatt et al. describes an optical collision detection scheme utilizing taps at each transceiver to divert signals to a single multiple channel optical receiver.
An in-band collision detection scheme is needed which minimizes false indication of collisions (no collision, but indication is present) and missed collisions (failure to indicate a collision).